A navigation system retrieves routes under respective retrieval conditions such as a condition that priority should be given to a time and a condition that priority should be given to a distance. The system then selects a route suitable for a driver's purpose from among the retrieved routes, and designates the selected route as a guide route so as to give driving assistance (refer to, for example, Patent Document 1).
Moreover, a navigation system acquires information on a traffic jam along a route from an information center. The system then designates a dynamic route, on which the traffic jam information is reflected, and gives driving assistance according to the dynamic route (refer to, for example, Patent Document 2).
In the system described in Patent Document 1, once a route suitable for a driver's purpose is designated as a guide route, driving assistance is given according to the designated guide route even when the guide route under way is determined to include a congested zone. In contrast, in the system described in Patent Document 2, when information is received to determine that driving is affected by a traffic jam included in a route under way, a dynamic route to reflect the traffic jam information may be designated so as to give driving assistance.
However, a retrieval condition under which a dynamic route is retrieved may not match a retrieval condition under which a route selected by a user has been retrieved. For instance, although an already designated guide route is retrieved with priority given to a reach time, the dynamic route may be retrieved with priority given to a distance. Otherwise, although the already designated guide route is retrieved with priority given to a distance, the dynamic route may be retrieved with priority given to a reach time.                Patent Document 1: JP-H7-91971 A        Patent Document 2: JP-H10-160498 A        